1. Field of the Invention
The present invention relates generally to processing digital signals, and more particularly, to a method for embedding digital information into an audio or sound signal in telecommunication systems.
2. Description of the Related Art
It is well known that sound waves are used in a data communication. The use of sound waves in telecommunication relates to short range data communication that does not use wireless or optical communication hidden to an observer. An example may be the use of a sound communication for exchanging digital information among mobile devices. One of the major advantages in the case of using such a communication type is that an upgrade of a conventional communication device is not required and typically only additional software is needed.
Various methods for solving problems associated with sound communication are disclosed in the conventional art. One of the methods for embedding an unobtrusive signal with digital information into an audio track is to add a spread spectrum signal having a level lower than a zero level to an audio signal as described by I. J. Cox, J. Kilian, T. Leighton and T. Shamoon, “A Secure, Robust Watermark For Multimedia”, Lecture Notes in Computer Science, Volume 1174/1996, pp. 185-206 (1996).
Another method for solving such problems may be “echo-modulation”. In this method, an echo on a low level is added to an audio signal, and the delay or the level of the echo is modulated according to digital information, as described by Gruhl. D., Lu, A, and Bender, W., “Echo Hiding,” Proceedings of the First International Workshop on Information Hiding, Cambridge, UK, May 30-Jun. 1, 1996, pp. 293-315.
US Patent Publication No. 2011/0144979 discloses a method for embedding digital information in an audio signal based on multicarrier digital modulation using the psychoacoustic characteristic of a human acoustic system.
A method based on a broadband signal (also referred to as a “spread spectrum signal”) with an amplitude lower than a zero level or based on digital modulation using psychoacoustic masking and a plurality of carrier waves generally has a higher data transmission rate than a method based on echo-modulation. The method undetectably embeds a digital information stream having data transmission rate of several kilobytes or more per second into an audio signal. However, due to a special characteristic of a human auditory system, such a method mainly uses high audible frequency, which provides a more noticeable frequency-time masking effect. Therefore, when a sound is transferred over the air, the high frequency quickly attenuates according to an increase in distance between a sound source and a receiver (a microphone), and in addition, the sound does not pass through a physical obstacle while transmitting the sound. As a result, such systems perform data transmission using sound over a considerably short distance (for example, 10 centimeters) and are generally applied to an application example in which a clear line-of-sight is secured between the sound source and a microphone.
Echo modulation is less sensitive to an obstacle between a sound source and a microphone and is appropriate for a data transmission through a sound over a relatively long distance (for example, several meters). On the other hand, this transmission type has defects such as a low processing rate (generally, several bits or several tens of bits per second) due to an overload of a microphone over a short distance, and sensitivity to noise and non-linear distortion.